Production of polyphenyls



United States Patent Ofiice 3,359,340 Patented Dec. 19, 1967 3,359,340 PRODUCTION OF POLYPHENYLS Clare R. Hand, Sanford, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Nov. 26, 1965, Ser. No. 510,031 3 Claims. (Cl. 260-670) This invention relates to a chemical process and it is particularly concerned with an improved method for making polyphenyls such as biphenyl and terphenyl.

Biphenyl and, to a lesser extent, terphenyl are conventionally manufactured by the noncatalytic pyrolysis of benzene. At the high temperature required by this pyrolytic condensation, usually about 800 C., there is normally considerable unwanted further condensation and decomposition resulting in the formation of tar, excessive noncondensable gas, and coke. These undesirable effects cause waste of benzene and make the separation of a pure or light colored product difi'icult and unduly expensive. Cooking in the reactor tubes also necessitates periodic shut-downs and burning out procedures.

Various expedients have been employed to inhibit the formation of carbon and tar in this process or to increase the yield of biphenyl. For example, one or the other of these objectives has been sought by employing additives to the benzene feed such as water, carbon dioxide, sulfur, and aliphatic organic compounds. While such additives have been found to offer some advantage, they also introduce obvious difficulties into the process including corrosion of equipment, complications in equipment design and product purification, and so on.

Additionally, reactor tubes of difierent materials of construction have been tried and some materials, particularly chromium alloys and copper, have been found to be useful in reducing tar and carbon formation.

It has now been found that in the process wherein benzene is thermally condensed to make polyphenyls such as biphenyl and terphenyl by subjecting its vapor to a pyrolyzing temperature, increased conversion of benzene and sharply reduced tar and carbon formation are both obtained when the benzene feed contains 0.1-5 percent by weight of benzoic acid. The presence of this additive in the benzene feed not only increases biphenyl production by to 150 percent or more, depending upon the conditions, but it also essentially eliminates tar and carbon formation at conventional process temperatures, thereby providing a crude pro-duct which is light yellow rather than black and which consists essentially of a solution of biphenyl in benzene with a small amount of terphenyl also present. Since the benzoic acid additive is ordinarily decarboxylated to benzene during the process, this additive is not found in the reaction product under preferred operating conditions but merely serves as additional feed as far as material balance and product purity are concerned.

It is known that some biphenyl is formed when benzoic acid is passed through a hot tube. However, the increased conversion caused by its presence in this new process is far above that which would be obtained even if all of the benzoic acid were converted to biphenyl, so this effect and particularly the effect of minimizing tar and carbon production are unexpected and could not have been predicted. For best results, 0.52 percent by weght of benzoic acid is added to the benzene feed. As the concentration of benzoic acid in the benzene feed to the process is raised above about 5 percent, an increasing amount of unconverted acid is found in the effluent prodnet. For the purpose of this invention, benzoic anhydride is the obvious equivalent of benzoic acid and it can be employed in its place if desired.

The process is run at a temperature Within the range known to cause conversion of benzene to biphenyl, i.e., about 600900' C. Preferably, the pyrolysis according to the present invention is carried out at 650850 C.

The benzene conversion varies according to the process temperature and inversely in proportion to the rate at which the benzene is passed through the reactor. This rate is perhaps best defined in terms of residence time, that is, the calculated average time taken for a molecule of benzene to pass through the pyrolysis reactor under process conditions. A residence time of 01-10 seconds has been found suitable and a time of 0.55 seconds is preferred. These residence times are similar to those of the prior art.

Best results are obtained when a copper surface is present in the reactor. The function of the copper appears to be as an additional aid in inhibiting tar and carbon formation. The copper may be present as the inner surface of the reactor or as a permeable bed within the reactor composed of pieces of copper or finely divided copper deposited on the surface of an inert support. Metallic copper or a substance which becomes copper during the process can be used. For example, the reactor may contain a bed of an inert material coated or impregnated with a copper compound which is reduced or decomposed to copper under process conditions.

Known additives or improvements other than the presence of copper can also be used in conjunction with the present invention. For example, water or carbon dioxide can be added to the benzene-benzoic acid feed and under some conditions somewhat improved results may thereby be obtained. Usually, however, such advantage is small.

EXAMPLE 1 The experimental pyrolysis equipment comprised three heating stages: a vaporizer consisting of a flask packed with glass wool and maintained at about 250275 C., a preheater which was a high-silica glass tube 1 x 14 inches electrically heated to 350400 C., and a reactor similar to the preheater but containing a 9 /240 inch bed of cut up 16 mesh copper screen and heated over this length at the pyrolysis temperature by an induction furnace. The liquid feed was introduced into the vaporizer by a metering pump, the vaporized feed then passed successively through the preheater and the reactor, and the condensable effluent was collected in a water-cooled flask vented through a cold trap in a solid carbon dioxide-acetone bath.

A solution of 1 percent by weight benzoic acid in benzene was made up and 361 g. of this solution was passed through the pyrolysis apparatus at a uniform rate over a period of 3 hours and at a reactor temperature of 680 C. The product collected in the water-cooled flask was a light yellow solution amounting to 356.4 g. Vapor phase chromatographic analysis of this product showed that it was essentially benzene containing 10 percent by weight of biphenyl and 1.0 percent of terphenyl. Distillation of a portion of this product confirmed this analysis. The isolated polyphenyl fraction was a light yellow crystalline solid. The cold trap contained 2.4 g. of condensate which was essentially pure benzene. No significant amount of benzoic acid or tar was found in the pyrolyzed product. The above values indicate a conversion of benzene to polyphenyl of 11.1 percent,

a 91 percent yield of biphenyl based on that conversion, and a 9 percent yield of terphenyl. The benzoic acid was assumed in these calculations to have been converted to benzene or polyphenyls. Recovered condensable products represented 100 percent of the theoretical quantity.

EXAMPLES 2-3 Using the apparatus of Example 1 except that the pyrolysis reactor was an open tube, two experiments were performed at a reactor temperature of 800 C. and a feed rate of 82.8 g. per hour. In the first experiment the feed was pure benzene; in the second the feed was benzene containing 1.14 percent by weight of benzoic acid. The results are listed in the following table.

Condensed Product Ex. Percent No. Benzoic in feed Percent Percent Percent Color Biphenyl Terphenyl Tar 2 0 13.6 4.1 2 Black. 3 1.14 14.9 4. 0 1 Lt. yellow.

EXAMPLES 4-6 Condensed Product Ex Percent Benzoic No Acid in Feed Percent Biphenyl Percent Terphenyl A trace of benzoic acid was present in the condensed product of Example 6.

I claim:

1. In a process for making a polyphenyl by subjecting benzene vapor to a pyrolytic temperature, the improvement wherein said benzene contains 0.l-5 percent by weight of benzoic acid.

2. The process of claim 1 wherein the benzene vapor is subjected to a pyrolytic temperature in the presence of copper.

3. The process of claim 1 wherein the temperature is 650-850 C.

References Cited UNITED STATES PATENTS 5/1934 Tschunkur et al 260-670 1/1939 Conover et a1 260670 30 PAUL M. COUGHLAN, JR., Primary Examiner.

C. R. DAVIS, Assistant Examiner. 

1. IN A PROCESS FOR MAKING A POLYPHENYL BY SUBJECTING BENZENE VAPOR TO A PYROLYTIC TEMPERATRUE, THE IMPROVEMENT WHEREIN SAID BENZENE CONTAINS 0.1-5 PERCENT BY WEIGHT OF BENZOIC ACID. 